A differential amplification circuit is used widely in various kinds of fields and a gain-variable differential amplification circuit is used for a certain purpose. For example, the frequency characteristics of the reception circuit of the differential amplification circuit that is used in a high-speed data reception circuit change by changing the gain. Because of this, the frequency characteristics are optimized by evaluating the error rate of received data and the eye pattern of a received signal and by adjusting the gain of the differential amplification circuit based on the evaluation results.
Hitherto, the gain of the gain-variable differential amplification circuit is changed generally by changing the resistance value of the resistor within the differential amplification circuit and desirably, the gain can be adjusted in accordance with a digital code from the viewpoint of usability. Consequently, the resistor within the differential amplification circuit is turned into a variable resistor in which a plurality of adjustment units including a switch and a resistor connected in series is connected in parallel, then gain adjustment is made by adjusting the resistance value by sequentially varying the number of switches that are turned on (brought into conduction) of the plurality of adjustment units.
Desirably, the gain-variable differential amplification circuit whose gain is set in accordance with a digital code has a wide adjustment range and at the same time, the gain changes in equal steps, i.e., changes linearly at fixed dB intervals in accordance with the value of the digital code. It is known that the gain of the differential amplification circuit changes in accordance with a predetermined expression. Consequently, the resistance values of a plurality of adjustment units are set and the number of switches that are turned on of the plurality of adjustment units is controlled in accordance with the value of the digital code so that the gain changes linearly in equal steps in accordance with the expression as the number of switches that are turned on is increased. Due to this, a differential amplification circuit whose gain changes linearly in equal steps in accordance with the value of the digital code is implemented. In this case, the resistance value of the adjustment unit is determined in correspondence to an amount of change in gain by which the gain is changed in accordance with the value of the digital code, and therefore, it is possible to determine the resistance value with ease.
In the above-described differential amplification circuit, as the value of the digital code changes by one, the number of switches that are turned on changes in order and, for example, when the gain reaches its maximum, all of the switches are turned on. In other words, when the gain is changed by one step, one resistor is connected in parallel, and therefore, one of the values of the digital code changes. Because of this, the differential amplification circuit requires the same number of switches as the total number of necessary steps of the gain.
The gain-variable differential amplification circuit is required to reduce the area of the circuit and if switches in the same number as the total number of steps of the gain, i.e., if adjustment units in the same number as the total number of steps of the gain are provided, the area of the circuit increases. Because of this, it is required to implement a comparatively wide adjustment range and small adjustment intervals (step widths) with a small number of switches in the gain-variable differential amplification circuit.
In the case where the resistance value, etc., is adjusted stepwise, implementing a comparatively wide adjustment range and small adjustment intervals with a small number of switches by combining coarse adjustment and fine adjustment is known. Consequently, it is easy to adjust the resistance by combining coarse adjustment and fine adjustment also in the above-described gain-variable differential amplification circuit.